1. Field of the Invention
This invention relates to a grating-type optical component, in which a grating is formed in specific areas of an optical waveguide, such as an optical fiber, and to a method of manufacturing the same.
2. Description of the Related Art
Along with the growth of information industries, the amount of information communicated is rapidly increasing, and high bit rate transmission fiber optics communication systems are becoming indispensable. In recent years, a wavelength multiplex transmission technique for transmitting signal light containing multiple different wavelengths through a single optical fiber has been studied as an approach to the high-speed large-capacity communication systems.
In connection with such wavelength multiplex transmission and communication systems, a study has been conducted for selectively reflecting a light component having a specific wavelength band from among the wavelength-multiplex transmission light in order to use this reflected light as a system monitoring light. A filter is generally used to selectively reflect the light component of a specific wavelength band, while the rest of the light components are transmitted through the fiber. In order to filter a specific light component, it has been proposed to form a grating directly on a selected area of an optical waveguide, e.g., a single-mode optical fiber. Such a grating-type optical component is also attracting a great deal of attention as one approach to a distribution compensation technique for achieving high bit rate transmission fiber optics communication.
A conventional single-mode optical fiber (or the optical waveguide) mentioned above has a core and a clad layer surrounding the core. This single-mode optical fiber has a step-type or rectangular distribution profile of index of refraction in the radial direction of the optical fiber, as shown by the solid line "a" in FIG. 6. The index of refraction is substantially constant in the core in the radial direction of the optical fiber, and the index of refraction of the clad layer surrounding the core is also constant, but has a value different from the core, in the radial direction. This means that the index of refraction of the fiber drastically changes at and near the boundary between the core and the clad layer in the radial direction. In this conventional single-mode optical fiber, the specific refraction differential .DELTA.1, of the core, relative to pure silica, is set greater than the specific refraction differential .DELTA.3, of the clad layer, relative to pure silica.
The specific refraction differential .DELTA.1, of the core, relative to pure silica, and the specific refraction differential .DELTA.3, of the clad layer, relative to pure silica, are defined by equations (1) and (2), respectively. EQU .DELTA.1=[(n.sub.C.sup.2 -n.sub.0.sup.2)/2*n.sub.c.sup.2 ]*100 (1)
.DELTA.3=[(n.sub.L.sup.2 -n.sub.0.sup.2)/2*n.sub.L.sup.2 ]*100 (2)
where n.sub.0 is the index of refraction of pure silica (i.e., silica; SiO.sub.2), n.sub.L is the index of refraction of the clad layer surrounding the core, and n.sub.c is the index of refraction of the core.
In the example shown in FIG. 6, .DELTA.1 is 0.3% and .DELTA.3 is 0%, which results in the rectangular refractive index profile indicated by the solid line "a". In this case, the clad layer of the single-mode optical fiber is made of pure silica (n.sub.L =n.sub.0).
A grating is formed on this grating-type optical component in order to produce a light-blocking region, whereby a part of the propagating light having a specific wavelength band, which is different from the signal transmission wavelength band, is blocked or reflected by the grating. In other words, signal light is transmitted through the optical fiber from the input terminal to the output terminal, while the light component having the wavelength of the prescribed light-blocking band is reflected back to the input terminal, which is then extracted as monitoring light.
The grating is formed generally by irradiating the optical waveguide (e.g., the optical fiber) with ultraviolet beams through a phase mask. The ultraviolet beams are diffracted by the phase mask and, therefore, the optical waveguide is illuminated by the diffraction light. Those areas on the core exposed to the ultraviolet beams become high refractive index areas because the ultraviolet beams excite a rare-earth element (e.g., germanium (Ge)) doped in the core, which causes the index of refraction of the illuminated areas to increase by an optical inductive effect. On the other hand, the non-exposed areas of the core become low refractive index areas because no change occurs in the index of refraction. The high-refraction areas and the lower-fraction areas are alternately formed along the longitudinal axis of the core. As the grating formation method, a holographic method, without using a mask, is also known, other than the phase mask method. With a holographic method, two coherent ultraviolet beams interfere with each other, and the optical waveguide is irradiated by the interference light, whereby a grating is formed. (Appl. Phys. lett., 62, 1035, 1993)
In wavelength multiplex transmission systems, it has been proposed to use the 1550 nm wavelength band as a signal transmission band, and use the light component having the wavelength of about 1650 nm as monitoring light.
This is because, in parallel with the development of wavelength multiplex transmission systems, optical fiber amplifiers using erbium doped fibers, which can amplify the optical signals (or the signal light), have also been developed in order to achieve high bit rate transmission optical communication systems, and because the gain regions of such erbium-added fibers are about 1550 nm. If the wavelength of the transmitted signal light is set to 1550 nm, the monitoring light must have some other wavelengths within the range below the cutoff wavelength (about 1700 nm). Therefore, 1650 nm is preferably chosen for the wavelength band of the monitoring light. If the wavelength of the monitoring light exceeds 1700 nm, a portion of the propagation light is lost, which is undesirable.
However, this conventional grating-type optical component using a single-mode optical fiber has some problems. If the light component having a wavelength of about 1650 nm is selectively reflected, a large transmission loss occurs at and near 1520 nm, as shown in FIG. 7A.
This wavelength band, at which such a large light-transmission loss occurs depends on the wavelength of the reflection band (or the light-blocking band) of the fiber grating. Meanwhile, it is generally known that the transmission loss is caused by a clad-mode coupling loss which results from a phenomenon that the propagation mode, in which the propagating light tends to propagate through the core passing through the fiber grating, is coupled with the reflection mode, in which a portion of the light reflected by the fiber grating oozes into the clad layer and propagates through the clad layer.
Accordingly, if the conventional grating-type optical component having a light-blocking band of about 1650 nm is used in a wavelength multiplex transmission system in order to reflect or extract the monitoring light, while the monitoring light is effectively extracted, the intensity of the transmitted signal light, whose wavelength is about 1550 nm, inevitably drops at and near 1520 nm.